The goal of this research proposal is to establish how crossing the blood-brain barrier (BBB) regulates chemokine receptor expression on peripheral blood mononuclear cells (PBMC) including T cells, B cells and monocytes. The research strategy utilizes a novel and unique model of the BBB in vitro, incorporating flow conditions. The overall hypothesis is that individual chemokine receptors are regulated differentially on different subsets of leukocytes, contingent on ligand availability; leukocyte lineage; and leukocyte activation state. Performance of this research will advance current knowledge in two ways: first, we will gain new information about chemokine receptor modulation under flow conditions as PBMC interact with the unique cerebrovascular endothelium; second we will be enabled to interpret chemokine receptor expression by tissue-infiltrating cells as revealed by immunohistochemistry studies of the inflamed human central nervous system (CNS). The Specific Aims are: 1. To establish how luminal 'arrest' chemokines modulate chemokine receptor expression on transmigrated cells. These studies will incorporate both PBMC from healthy volunteers and from MS patients receiving natalizumab, in whom chemokine receptor engagement with ligand is uncoupled from arrest and transmigration. 2. To determine how abluminal 'transmigration' chemokines regulate chemokine receptor expression on transmigrated cells both at the protein and mRNA levels. This knowledge will identify specific chemokine receptors as salient therapeutic targets to treat neuroinflammatory diseases. PUBLIC HEALTH RELEVANCE: The blood-brain barrier (BBB) consists of specialized vessels which course directly through, and nourish, the brain. These specialized vessels limit the entry of noxious substances and promote access for beneficial components such as glucose. In addition, the BBB restricts entry of immune cells into the brain both during defense against germs and in the setting of inflammatory diseases such as multiple sclerosis (MS). The 'bottom line' here is that immune cells use a special set of 'rules' to enter the brain across the BBB. These 'rules' correspond to molecules on the immune cell surface and identifying these molecules is critical if we are to develop safe, effective ways to reduce harmful immune cell entry and promote immune-cell entry when it's beneficial. In this research project, we propose to utilize a newly-developed in-vitro (tissue culture) model of the BBB. This model incorporates the major elements of the BBB: specialized blood vessel cells, flowing culture medium (mimicking blood) and human immune cells and molecules. Sophisticated analytic methods will be used to study the cells, which can be recovered after they've crossed into the 'brain' side of the system. We will use this system to delineate the 'rules' which subsets of immune cells use to cross the BBB, and we will also test which molecules remain on the cell surface after crossing. These results will add new knowledge about this important process, and will also help us to interpret studies done in previous years. This knowledge is essential as we move towards safely and effectively blocking immune-cell movement into the brain during MS and other diseases.